The invention concerns a method for regulating the rotation speed of a motor, and a motor for carrying out such a method.
Digital systems for sensing and regulating the rotation speed of a rotating object (xe2x80x9crotorxe2x80x9d) are based on the fact that such a rotor generates one or more signals, e.g. needle pulses, pulse edges, or the like, at each revolution. These signals are referred to hereinafter as xe2x80x9cevents.xe2x80x9d Measurement of a rotation speed is based on measuring the time offset l between such xe2x80x9ceventsxe2x80x9d or counting the number of such xe2x80x9ceventsxe2x80x9d that occur per unit time. In a regulation process, this rotation speed information is used to keep the rotation speed of a motor at a defined value. A xe2x80x9cmotorxe2x80x9d (prime mover) is understood here to be any kind of motor, e.g. an internal combustion engine, electric motor, pneumatically or hydraulically operating is motor, turbine, etc. The invention is preferably applied to regulating the rotation speed of electric motors.
Since the number of such xe2x80x9ceventsxe2x80x9d per unit time increases with increasing rotation speed, processing them in a digital system requires more calculation time as the rotation speed becomes greater, so that the calculation time available for other processes decreases.
It is an object of the invention to make available a new method for regulating the rotation speed of a motor, and a motor for carrying out such a method.
According to a first aspect of the invention, this object is achieved by [a method according to claim 1.] calculating a first numerical frequency value representing actual rotation speed and a second numerical frequency value representing a target rotation speed, and processing these values in a digital motor controller in order to regulate the rotation speed toward the target value. Because two digital frequency values are calculated, it is possible to subject one or even both of said frequency values to a mathematical transformation and thus, by means of the digital controller, to create a desired relationship between the first and the second frequency value. Often one of the digital frequency values, or even both, will be multiplied by a constant factor, which in digital terms is often possible by decimal-point shifting and easily makes possible an electronic gearbox; other relationships are also conceivable, however, for example an exponential function or any other mathematical function in accordance with the requirements of the particular drive system task.
A particularly advantageous embodiment of the invention is [the subject matter of claim 5] to define a series of four instants which span multiple xe2x80x9ceventsxe2x80x9d of the frequency signal, and to use a count of events between the second and fourth instants to calculate the frequency. Because multiple xe2x80x9ceventsxe2x80x9d of the frequency signal can be sensed with one measurement, an averaging takes place upon measurement of the frequency information. This is because if the frequency is ascertained by means of a plurality of short measurements, it is often necessary to determine a moving average from multiple measured values by calculation. With the invention, on the other hand, an average is obtained without additional calculation, thus additionally relieving the load on, for example, a microprocessor or microcontroller, both hereinafter abbreviated as xcexcC. And despite the approximate definition of a measurement duration by means of the predetermined instants, in the method the actual measurement takes place exactly between two events of the frequency signal. This has proven particularly advantageous because measurement exactly between two defined events of a frequency signal makes possible particularly high accuracy.
Another advantageous development of the invention is [the subject matter of claim 8] to count xe2x80x9ceventsxe2x80x9d which happen a fixed number of times per revolution of the rotor. The result of this is that, upon measurement of a rotation speed datum in the context of a rotor having a fixed number of xe2x80x9ceventsxe2x80x9d per rotor revolution, measurement always occurs between such xe2x80x9ceventsxe2x80x9d which are associated with the same rotational position of the rotor, i.e. complete revolutions are measured. This is of interest in particular for rotors which comprise multiple xe2x80x9cmarksxe2x80x9d for generating the rotor position signal, which marks do not have identical angular spacings. The result of the development, [according to claim 8] using such a fixed number of events per revolution, is thus that measurement always occurs at the same point on the rotor, so that the exactness of the angular spacings is immaterial. High measurement accuracy results therefrom. xe2x80x9cMarksxe2x80x9d are understood here also to mean invisible marks, e.g. a pattern in the magnetization of a permanent-magnet rotor.
Another advantageous development of the invention is [the subject matter of claim 12] to have the fourth instant of a first measurement correspond to the second instant of a second measurement. A frequency datum is measured continuously by means of such a method, since the next measurement seamlessly follows the present measurement. In the vicinity of each predetermined instant, a present measurement is ended and a new measurement is begun. If the predetermined instants each occur after a time T_A, a measurement then takes place, on average, after a time T_A in each case. Processing of the measurements, for example in a xcexcC, thus takes place at regular intervals that are independent of the rotation speed. The xcexcC is thereby under a uniform load for these tasks at all rotation speeds. Since in this case there is no pause between the individual measurements, this method is particularly suitable if each individual revolution of the rotor must be taken into account for measurement or regulation, for example as is necessary in motors with high-accuracy controllers or in stepping motors.
The invention furthermore concerns a method according to [claim 16] which the remainder from a first measurement is used in the calculation for a following measurement. The result of taking into account the remainder in the subsequent measurement is that, in the context of a calculation of the frequency information by division, no frequency information is lost due to rounding. This results very advantageously in an increase in the accuracy of the measurement, and is very advantageous in the context of complex control processes that require high precision. Two or more motors can thus run synchronously with one another by the fact that the same frequency signal is delivered to all said motors, i.e. without a higher-level controller. Systems having higher-level controllers are relatively slow, have long initial transients, and are susceptible to oscillations especially at low rotation speeds. A method according to the invention, on the other hand, operates quickly because only a few program steps are needed to take the remainder into account. A method according to the present invention can also easily be switched over to different parameters, e.g. to a different xe2x80x9cgearbox factor,xe2x80x9d different control parameters, etc.
The preferred use of a ring counter [according to claim 25] to measure the time between first and second instants, and the development [according to claim 26] in which the ring counter counts continuously, have the advantage that the termination of one measurement and the start of the subsequent measurement are simultaneous because the ring counter is always running, so that no errors can occur due to measurement delay; and any delays (resulting, for example, from the simultaneous occurrence of two interrupts) are subsequently immediately compensated for, since the time of the subsequent measurement cycle automatically becomes slightly too short.
This makes possible seamless sensing of the rotation speed and an average rotation speed sensing error that is close to zero, since not a single bit is lost during measurement. A ring counter of this kind usually has a so-called pre-scaler with which it is possible to set the frequency at which said counter counts. By setting this function on a bit-by-bit basis, the ring counter can be clocked at various fixed multiples of the quartz oscillator frequency of the xcexcC. The counting frequency and resolution of the ring counter can thereby be optimized for the application. This setting can be accomplished by way of a parameter that is stored in the motor in a nonvolatile memory. The resolution of the ring counter can thereby be optimized for different rotation speed ranges using different parameters.